Superconducting magnet coils allow extremely energy-efficient generation of strong and temporally constant magnetic fields, since said coils can be operated entirely without, or at least with very small, ohmic losses. The electrical current-carrying capacity of a superconductor is specified by its critical current Ic. If the electrical current in the conductor exceeds the value of Ic, a phase transition to a normally conducting state occurs, in which state the current no longer flows without resistance.
In an isotropic superconductor, the current-carrying capacity depends on the strength of the magnetic field to which said superconductor is exposed, but not on the direction of the magnetic field in a plane perpendicular to the conductor axis. In contrast, in an anisotropic superconductor, the current-carrying capacity is also influenced by the angle of the magnetic field relative to marked directions of the conductor, usually crystallographic directions. This is the case for example in high-temperature superconductors (HTS) such as (Re)BCO or Bi-2223, the underlying crystalline structure of which has a two-dimensional character. The critical current of a strip-like high-temperature superconductor (HTS) is therefore typically lower in a magnetic field perpendicular to the strip plane than in a field in parallel with the strip plane.
In a cylindrically symmetric magnet coil wound from strip-like HTS, this generally leads to the current-carrying capacity of the coil being limited at the axial ends of the winding stack, since the radial components of the magnetic field are greatest here.
In the following, a cylindrically symmetric magnet coil that is layer-wound from an anisotropic superconductor will be considered, the current-carrying capacity of which coil is more strongly suppressed by the field components produced by the coil in the radial direction than by those in the axial direction. “Layer-wound” means that successive windings along the superconductor are wound, mainly in layers, side-by-side along the axis of symmetry, it being possible for each layer to be assigned a constant radius. This is in contrast with so-called pancake coils, in which successive windings are wound over one another mainly radially.
DE 102 02 372 B4 (reference [1]) or U.S. Pat. No. 6,774,752 B2 (reference [2]) disclose solenoid-like coil sections as a solution, which coil sections are characterized in that the radially innermost coil section is wound with a strip-like superconductor, onto a coil support that protrudes, at least at an axial end, in the axial direction beyond the winding stack of the radially adjacent coil section, and in that the strip-like superconductor is guided tangentially outwardly, on this side, into a region of reduced magnetic field strength, and ends in at least one electrical connection point. In addition, the following solutions are disclosed as specific embodiments. In the region having reduced magnetic field strengths, two strip-like superconductors are interconnected and a plurality of strip-like superconductors are wound in one coil section. In addition, groove-like depressions in the surfaces of the bobbin for strip guidance are disclosed, as well as flexible mats and partial shells having depressions for defined guidance of the adjacent winding layers.
A disadvantage of these solutions is that there are no axial regions having different strip-like superconductors.
EP 2 906 961 B1 (reference [3]) discloses the following solution.
Strip-like HTS pieces are interlinked (soldered in series) within the winding stack, each linked strip piece being connected to two further strip pieces in each case, such that the further strip pieces together substantially overlap the overall length of the linked strip piece. FIG. 11 of this document discloses, as an example, a coil section comprising three portions in the axial direction, the edge regions being formed by a single strip and the central region being formed by a double strip.
This solution has multiple disadvantages. The soldering process within the coil is difficult, in particular, very long strip lengths have to be soldered in order to achieve low-ohmic resistance. The soldered connections (joints) are not superconducting. Problems of homogeneity may arise due to the variations in thickness of the strips in the soldered region. It is difficult to apply the electrical insulation at the soldering points in a uniform manner.
DE 10 2004 043 987 B3 (reference [4]) describes how a strip-like superconductor is guided from one sub-chamber into another sub-chamber via a single-layer transfer winding, over the frustoconical lateral surface of the first separating body, a second separating body being provided that extends the first separating body, in the notch region, radially outwardly towards a circular cylinder, the single-layer transfer winding being arranged between the separating bodies. The disadvantage of this solution is that the separating body is intended to define a notch region and therefore covers a large axial region. There are four windings in FIG. 1 of said document.
For coil assemblies that are wound with “double pancakes”, the approach using different conductor widths according to U.S. Pat. No. 9,117,578 B2 (reference [5]) can be applied, the wider conductors having higher current-carrying capacity due to being positioned at the edge of the coil. Coil assemblies of this type are poorly suited to NMR applications both due to the too low field stability (large number of soldered connections, and therefore relatively high resistance) and due to the inadequate field homogeneity (current distribution is spatially too uneven).